Loudspeakers dc protection - question about caps suitability - diyAudio
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Old 6th December 2015, 12:03 AM   #1
00940 is offline 00940  Belgium
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Default Loudspeakers dc protection - question about caps suitability

I'm planning to use a loudspeakers protection pcb based on the esp 33 project.

The project uses 10uF bipolar caps after 100K resistors as input filter. Would there be any downside in using 10uF/25V X7R caps in that position ?

There's little AC at that point (max 250mV at 20hz) and little dc too (max 2.5V).
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Old 6th December 2015, 08:40 AM   #2
AndrewT is offline AndrewT  Scotland
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Don't use X7R for signal duty.
They are ideal for local decoupling duty.

The 100k separate the distorting effect of the ceramic from the previous audio stages, but I would not.

You will find that the DC detect circuit works with a very wide range of filter turn over frequencies.
Try increasing the 100k to 330k and reducing the 10uF NP to 1uF, or 2u2F, or 3u3F plastic film. MKT/MKS is OK.
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Old 6th December 2015, 09:35 AM   #3
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I wouldn't use X7R for coupling duty. But this filter is hardly critical. The normally used bipolar cap isn't exactly a precision cap.

The datasheet isn't very explicit however on the variation of capacitance vs AC/DC signals. They only give two curves, for DC with a 01uf/50v cap and for AC with a 10uF/6.3V.

- I'm not too worried about DC: under 20% of the voltage rating, the capacitance is apparently equal to the nominal value. So that's fine.
- The variations with AC signal are more drastic. But is there a way to extrapolate what it would give in this particular situation and with a 25V rated cap ? If the variations are in a +/-10% range, it's fine.

PS: caps are grm31 from murata.
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Old 7th December 2015, 10:25 AM   #4
00940 is offline 00940  Belgium
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Murata has quite an amazing website when you start digging into it.

I've browsed through their detailled specs and the 1206 x7r 10uF/25V has pretty poor AC voltage characteristics. They however have a 1210 x7r 10uF/35V which is really pretty good. Considering those graphs, I see no reason not to use the 10uF/35V. Well, there's one: the cost is the same as a 3.3uF wima mks.

The 25V:
Click the image to open in full size.

The 35V (GRM32ER7YA106KA12):
Click the image to open in full size.
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File Type: png 35v.PNG (14.7 KB, 146 views)
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Old 8th December 2015, 01:05 PM   #5
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Unless you are placing the cap. near a heatsink, what's the problem with an economical BP electrolytic? It's not a critical performance item, so what are you trying to do - SMD only or everlasting parts? I can't see the point as these will outlast 99% of DIY projects.
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Old 8th December 2015, 02:51 PM   #6
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Small bipolars aren't stocked by the suppliers I use for most projects (Reichelt). It's more a question of convenience and avoiding extra shipping charges.
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Old 8th December 2015, 03:03 PM   #7
AndrewT is offline AndrewT  Scotland
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Bi-polar electrolytics only come in small values,
typically 1uF to 470uF.

Try a different retailer.
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Old 8th December 2015, 03:23 PM   #8
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Which is exactly what I want to avoid.
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Old 8th December 2015, 03:29 PM   #9
AndrewT is offline AndrewT  Scotland
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then try using back to back and/or front to front polar electrolytics.
They are not quite as good, but they are far better than many here allege.

I think it is Marsh that did distortion comparison measurements of polars, bipolars and back to back versions.

I also tried a back to back pair in parallel to a front to front pair.
4 off 10uF give a good 10uF bipolar, that when there is no AC voltage and no DC voltage across them, creates no distortion of the wanted audio signal.
I could not tell there was a capacitor in the circuit.
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Old 8th December 2015, 03:33 PM   #10
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What is the relevance of distortion in this case ? I'm not designing a servo or an audio filter. As long as variations in capacitance are kept under +/-10% in the operating conditions of those caps, the filter will fullfill its role.
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